1. Field of the Invention
The Present invention relates to bar code readers generally and, more particularly, to a novel bar code reader which permits reading bar codes of varying size over a wide range of distances, while allowing good bar discrimination regardless of such size or distance.
2. Background Art
Bar codes have found application in a wide variety of applications as an information source, typically for digital processors. Such bar codes are used at point-of-sale in merchandising for pricing and inventory count, in controlled personnel access systems, and in manufacturing for work-in-process and inventory control, to name only a few applications. The bar codes themselves comprise a series of parallel lines, typically in the range of about 1/8" to 1" in height and from about 1 to 50 mils in thickness, arrayed on a contrasting background. The lines may variously have unequal spacings and/or unequal thicknesses, with the variations in spacing and/or thickness determined by the information "stored" in the bar code. A bar code is "read" by serially illuminating the bars, with the bars absorbing light and the background reflecting light. The resulting pattern of reflection and nonreflection is sensed by a light detecting device which provides input to the digital processor. The bar code reader may be of the type that is passed over the bar code or of the type with which the bar code is moved passed the bar code reader.
There are two widely used types of bar codes: (1) the high-density type, the narrowest element of which is 7 mils wide and which includes 10 characters per inch for Code 39, and (2) the low-density type, the narrowest element of which is 30 mils wide and which includes 3 characters per inch for Code 39. The former is typically read at close range, while the latter is typically read at a distance.
Known bar code readers serially scan bar codes at a fixed rate of sweep. As a bar code is scanned, there is generated a number of pulse counts per bar, derived from a fixed clock signal, the number of counts being directly proportional to the width of a particular bar and, conversely, the number of counts between bars being directly proportional to the width of a particular space. Since the rate of sweep through the scan angle is fixed, the scan rate across a bar code that is positioned close to the bar code reader will be less than the scan rate across a bar code that is positioned farther from the bar code reader. It can be understood, therefore, that 3 counts for a given bar may be generated when, say, a high-density bar code is being read at a distance close to the bar code reader; but, if the same bar is at a farther distance from the bar code reader, only 1, or even less, count per bar may be generated because of the greater scan rate at that distance. As a result, that bar code reader would be unsuitable for reading that bar code at that farther distance. Likewise, when trying to read a low-density bar code having wide elements at close range, the high counts obtained may overload the decoding circuitry in the bar code reader. Consequently, it is necessary, in many cases, to provide a plurality of bar code readers having different fixed parameters in order to be able to read bar codes at different distances and/or the operator must adjust the position of the bar code reader relative to the bar codes.
A further disadvantage of present bar code readers is that they employ a fixed scan angle. Consequently, when one attempts to use such a bar code reader to read a bar code which has other bar codes in proximity to it, the bar code reader may scan portions of two or more bar codes. In some cases, this may be of no consequence, since the bar code reader will decode a scanned bar code only when it detects the quiet zones on either end of the encoded information. In other cases, however, portions of two or more bar codes may be scanned before the reader recognizes the quiet zones and, therefore, a false reading is obtained. One common method of trying to avoid this problem is to hold the bar code reader such as to place the plane of the scan at an angle to the axis of the bar code so that at least bar codes in line with the one being read will not be scanned. Another method of trying to avoid this problem is to provide the bar code reader with a fixed light source in addition to the scanning light source. When the operator wishes to read a bar code, he switches to the fixed light source to assure where the bar code reader is pointing, then switches to the other light source for scanning. Either of these methods somewhat improves the accuracy of bar code scanning, but both add additional time to the process.
Accordingly, it is a principal object of the present invention to provide a bar code reader which can read a range of bar code sizes over a range of distances.
Another object of the invention is to provide such a bar code reader which employs relatively conventional components and is economical to construct and easy to use.
An additional object of the invention is to provide such a bar code reader which improves the accuracy of bar code reading when there are other bar codes in proximity to the one being read.
Other objects of the present invention, as well as particular features and advantages thereof, will be apparent from the following description and the accompanying drawing figures.